Title: Measurements and simulations of mixing and autoignition of an n-heptane plume in a turbulent flow of heated air

Abstract

The autoignition of a gaseous n-heptane plume in heated turbulent air has been investigated experimentally and numerically with the conditional moment closure and a CFD code. It has been demonstrated that, consistent with previous experimental results for hydrogen and acetylene, the increased scalar dissipation rate created by faster co-flowing air delays autoignition, as revealed by a disproportionate increase of ignition length with air velocity. The predicted mean and variance of the mixture fraction, the mixture fraction PDF and the conditional scalar dissipation rate are in good agreement with experimental results obtained with acetone-tracer PLIF. The first-order, spatially averaged CMC model reproduces the experimental trends quite well, despite the neglect of conditional fluctuations and spatial dependence of the conditional averages. This is attributed to the fact that for a significant period of time before autoignition the conditional scalar dissipation rate at the most reactive mixture fraction is much smaller than the critical value above which autoignition is precluded. (author)

Citation Formats

Markides, C.N., De Paola, G., and Mastorakos, E.. Measurements and simulations of mixing and autoignition of an n-heptane plume in a turbulent flow of heated air. United States: N. p., 2007.
Web. doi:10.1016/J.EXPTHERMFLUSCI.2006.04.008.

Markides, C.N., De Paola, G., & Mastorakos, E.. Measurements and simulations of mixing and autoignition of an n-heptane plume in a turbulent flow of heated air. United States. doi:10.1016/J.EXPTHERMFLUSCI.2006.04.008.

Markides, C.N., De Paola, G., and Mastorakos, E.. Sun .
"Measurements and simulations of mixing and autoignition of an n-heptane plume in a turbulent flow of heated air". United States.
doi:10.1016/J.EXPTHERMFLUSCI.2006.04.008.

@article{osti_20864944,
title = {Measurements and simulations of mixing and autoignition of an n-heptane plume in a turbulent flow of heated air},
author = {Markides, C.N. and De Paola, G. and Mastorakos, E.},
abstractNote = {The autoignition of a gaseous n-heptane plume in heated turbulent air has been investigated experimentally and numerically with the conditional moment closure and a CFD code. It has been demonstrated that, consistent with previous experimental results for hydrogen and acetylene, the increased scalar dissipation rate created by faster co-flowing air delays autoignition, as revealed by a disproportionate increase of ignition length with air velocity. The predicted mean and variance of the mixture fraction, the mixture fraction PDF and the conditional scalar dissipation rate are in good agreement with experimental results obtained with acetone-tracer PLIF. The first-order, spatially averaged CMC model reproduces the experimental trends quite well, despite the neglect of conditional fluctuations and spatial dependence of the conditional averages. This is attributed to the fact that for a significant period of time before autoignition the conditional scalar dissipation rate at the most reactive mixture fraction is much smaller than the critical value above which autoignition is precluded. (author)},
doi = {10.1016/J.EXPTHERMFLUSCI.2006.04.008},
journal = {Experimental Thermal and Fluid Science},
number = 5,
volume = 31,
place = {United States},
year = {Sun Apr 15 00:00:00 EDT 2007},
month = {Sun Apr 15 00:00:00 EDT 2007}
}

The ignition delay times of mixtures containing 35% n-heptane and 65% toluene by liquid volume at room temperature (i.e., 28% n-heptane/72% toluene by mole fraction) were determined in a high-pressure shock tube in the temperature range 620{<=} T{<=}1180 K at pressures of about 10, 30, and 50 bar and equivalence ratios, {phi}, of 0.3 and 1.0. The equation {tau}/{mu}s=9.8 x 10{sup -3} exp (15,680 K/T)(p/bar){sup -0.883} represents the data for {phi}=0.3 in the temperature range between 980 and 1200 K. At lower temperatures no ignition was found at 10 bar within the maximum test time of 15 ms, whereas formore » 50 bar, a reduced activation energy was observed. A pressure coefficient of -1.06 was found for the data with {phi}=1.0. No common equation for the data at {phi}=1.0 could be found analogous to that for {phi}=0.3 because the ignition delay times show no Arrhenius-like behavior. A comparison with ignition delay times of n-heptane/air and toluene/air for {phi}=1.0 and 30 bar shows that the values of the mixture of the two components are between the values of the single substances. Furthermore, the results confirm the negative temperature coefficient behavior found for the mixtures at 30 and 50 bar, similar to n-heptane/air. A comparison for the other pressure and equivalence ratio values of this study was not possible because of the lack of data for pure toluene. These experimental data have been used in the development of a chemical kinetics model for toluene/n-heptane mixtures as described in a companion paper. (author)« less

In our paper, two- and three-dimensional direct numerical simulations (DNS) of autoignition phenomena in stratified dimethyl-ether (DME)/air turbulent mixtures are performed. A reduced DME oxidation mechanism, which was obtained using rigorous mathematical reduction and stiffness removal procedure from a detailed DME mechanism with 55 species, is used in the present DNS. The reduced DME mechanism consists of 30 chemical species. This study investigates the fundamental aspects of turbulence-mixing-autoignition interaction occurring in homogeneous charge compression ignition (HCCI) engine environments. A homogeneous isotropic turbulence spectrum is used to initialize the velocity field in the domain. Moreover, the computational configuration corresponds to amore » constant volume combustion vessel with inert mass source terms added to the governing equations to mimic the pressure rise due to piston motion, as present in practical engines. DME autoignition is found to be a complex three-staged process; each stage corresponds to a distinct chemical kinetic pathway. The distinct role of turbulence and reaction in generating scalar gradients and hence promoting molecular transport processes are investigated. Then, by applying numerical diagnostic techniques, the different heat release modes present in the igniting mixture are identified. In particular, the contribution of homogeneous autoignition, spontaneous ignition front propagation, and premixed deflagration towards the total heat release are quantified.« less